Monitoring a drive on Windows is pretty easy, as a drive usually has a drive-letter assigned (for example C:).
Here I'm using NSClient++ running as agent on the Windows host while on the monitoring server I use check_nt to query the agent:

But what about drives that appear in the disk management but are not assigned with a drive letter but are rather mounted as a folder?
In this example we have the classical C: drive for the Windows OS and an additional D: as data partition. But as you can see in the Disk Management UI, Disk 2 (named SQL_Data_DEV001) and Disk 3 (named SQL_Log_DEV001) have no drive letter assigned.

And here comes the good news: The NRPE command check_drivesize (internally configured within the NSClient agent, no need to define this command somewhere) also allows mounted volumes. From the NSClient++ documentation:

To check the size of a mounted volume (c:\volume_test)[...]

According to the documentation, only the mount-path is needed. Let's try that: